I had a lot of information on my leads in "Hushabye Prince" before it started inasmuch as it was relevant to their behavior. Birthdays and such I decided as I need them, because they don't affect my leads personalities, but things like favorite foods, hobbies, bad habits, and at least basic details on their family backgrounds I do.
I also know a lot about their worst fears, wants and desires and things like that before I started.
With "Damsel in the Red Dress" because I wrote it for the True Love On Tapas competition I only learned about a month from the deadline I didn't plan much in advance. That said, I did still put a lot of details into their backgrounds. I know their hobbies, what their childhoods were like, relationship statuses in the past (my male lead has never dated anyone, but no, it's not because he's a sad unpopular boy)
I love writing backstories, so even if I never get to share them, I usually write them with a pretty decent amount of detail. I could tell you how my female lead in "Hushabye Prince" parents met, how her father lost his hearing, how her mother came to the United States. Etc. but there probably won't be space for it in the story lol.